Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) specifications starting with LTE Release 11 utilize carrier aggregation. Carrier aggregation is a technique by which multiple frequency bands, which for LTE are multiple 20 Megahertz (MHz) frequency bands, are aggregated for downlink or uplink transmission. A 3GPP LTE cellular transceiver that supports carrier aggregation can be configured to simultaneously transmit in multiple frequency bands and/or simultaneously receive in multiple frequency bands.
Currently, there is high interest in a single-chip cellular transceiver that supports carrier aggregation. One of the many design challenges for such a transceiver is Local Oscillator (LO) frequency planning. LO frequency planning refers to the selection of the LO frequencies. In order to operate according to a carrier aggregation scheme, a single-chip cellular transceiver includes multiple Phase-Locked Loops (PLLs) to generate LO signals necessary for downconversion and/or upconversion for multiple frequency bands. The number of different frequency band combinations that have to be supported by the single-chip cellular transceiver for carrier aggregation is large and new combinations are being introduced all the time. Many of these combinations require multiple PLLs to be enabled simultaneously.
One challenge in frequency planning that is particularly problematic for a single-chip carrier aggregation cellular transceiver comes from the fact that the Controlled Oscillators (COs) in the PLLs are sensitive to interference. For example, two PLLs running at the same frequency, or approximately the same frequency, interfere with each other. This interference degrades the noise performance of the PLLs. This same problem occurs if the PLLs run at frequencies that have a harmonic relation (i.e., a harmonic of the LO frequency of one PLL is the same as or approximately the same as the LO frequency of another PLL). In a receiver, this interference results in degradation of throughput due to phase noise sidebands in the LO signal used for downconversion. During downconversion (i.e., mixing), these phase noise sidebands mix parts of the received signal on top of itself. Still further, interference to a CO in a PLL is not limited to interference from a CO of another PLL. For example, in a wireless transmitter or transceiver, a harmonic of a modulated transmit signal may interfere with the CO of a PLL used to, e.g., generate a clock signal for upconversion in the transmitter or downconversion in the receiver.
Commonly owned and assigned U.S. patent application Ser. No. 14/259,485 discloses systems and methods for mitigating crosstalk between COs of PLLs. In this application, the systems and methods are used to compensate for a Continuous Wave (CW) crosstalk signal (interference) from the CO of one PLL to the CO of another PLL in the system. In general, a compensation signal is generated at an offset frequency that is approximately equal to an offset between the frequency of a first CO and the frequency of a second CO. The compensation signal is applied to the first CO to thereby compensate for a crosstalk signal from the second CO to the first CO. One issue with these systems and methods is that they provide compensation for only the CW crosstalk signal. However, in some systems, there may be modulated interferers and, as such, there is a need for systems and methods for compensating for modulated interferers.
Commonly owned and assigned U.S. patent application Ser. No. 14/264,506 discloses systems and methods for mitigating interference in a LO output signal generated by a PLL. In one embodiment, a system includes a PLL and an error compensation subsystem. Based on the output signal of the phase detector in the PLL, the error compensation subsystem applies a phase rotation to a signal derived from the LO output signal to compensate for a phase error in the signal resulting from a phase error in the LO output signal indicated by the phase detector output signal. Since the bandwidth of the phase detector is substantially larger than the bandwidth of the PLL, these systems and methods enable compensation for interferers that fall outside of the bandwidth of the PLL but inside the bandwidth of the phase detector. These interferes may be modulated signals. However, these systems and methods are limited by the Signal-to-Noise Ratio (SNR) of the phase detector. This means that the systems and methods disclosed in Ser. No. 14/264,506 can be used in frequency areas where the phase detector noise is less than the PLL noise. Thus, there is a need for systems and methods for compensating for modulated interferers that are not limited by the SNR of the phase detector of the PLL.